Rollup merge of #123806 - joboet:advanced_overflow, r=Amanieu

Panic on overflow in `BorrowedCursor::advance`

Passing `usize::MAX` to `advance` clearly isn't correct, but the current assertion fails to detect this when overflow checks are disabled. This isn't unsound, but should probably be fixed regardless.

library/std/src/io/tests.rs

@@ -209,6 +209,15 @@ fn read_buf_exact() {
     assert_eq!(c.read_buf_exact(buf.unfilled()).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
 }
 
+#[test]
+#[should_panic]
+fn borrowed_cursor_advance_overflow() {
+    let mut buf = [0; 512];
+    let mut buf = BorrowedBuf::from(&mut buf[..]);
+    buf.unfilled().advance(1);
+    buf.unfilled().advance(usize::MAX);
+}
+
 #[test]
 fn take_eof() {
     struct R;